Method and component for forming an embedded resistor in a multi-layer printed circuit

ABSTRACT

A component for use in forming a multi-layer printed circuit comprised of a film substrate formed of a first polymeric material. At least one layer of a flash metal is applied to a first side of the film substrate, and at least one layer of copper is applied on the layer of flash metal. A discrete area of a resistive material is disposed on a second side of the film substrate.

FIELD OF THE INVENTION

[0001] The present invention relates generally to printed circuits, andmore specifically, to a method and component for manufacturing embeddedresistive elements in printed circuit boards.

BACKGROUND OF THE INVENTION

[0002] In recent years, printed circuit components have become widelyused in a variety of electronic devices. Of particular interest aremulti-layer printed circuit board laminates which have been developed tomeet the demand for miniaturization of electronic components and theneed for printed circuit boards having a high density of electricalinterconnections and circuitry. In the manufacture of multi-layerprinted circuit boards, conductive foils, which are usually copperfoils, are secured to opposite sides of a core which is conventionally areinforced or non-reinforced dielectric. (Throughout this specification,the use of the term “core” is meant to include any one of a variety ofcore materials, all of which may be reinforced or non-reinforced and mayinclude an epoxy, polyester, polyimide, a polytetrafloroethylene, and insome applications, a core material which includes previously formedprinted circuits).

[0003] The process includes one or more etching steps in which theundesired or unwanted copper is removed by etching away portions of theconductive foil from the laminate surface to leave a distinct pattern ofconductive lines and formed elements on the surface of the etchedlaminate. The etched laminate and other laminate materials may then bepackaged together to form a multi-layer circuit board package.Additional processing, such as hole drilling and component attaching,will eventually complete the printed circuit board product.

[0004] The trend in recent years has been to reduce the size ofelectronic components and provide printed circuit boards havingmulti-chip modules, etc. This results in a need to increase the numberof components, such as surface-mount components provided on the printedcircuit board. This in turn results in a so-called “densely populated”or simply “dense” printed circuit board. A key to providing a denselypopulated printed circuit board is to produce close and fine circuitpatterns on the outer surfaces (i.e., the exposed surfaces) of theresulting multi-layer printed circuit board. The width and spacing ofconductive paths on a printed circuit board are generally dictated bythe thickness of the copper foil used thereon. For example, if thecopper foil has a thickness of 35 μm (which is a conventional 1-ouncefoil used in the manufacture of many printed circuits), exposing theprinted circuit board to an etching process for a period of time toremove such a foil thickness will also reduce the width of the sideareas of the printed circuit path in approximately the same amount. Inother words, because of the original thickness of the copper foil, aprinted circuit board must be designed to take into account that anetching process will also eat away the sides of a circuit path (i.e.,undercut a masking material). In other words, the thickness of thespacings between adjacent circuit lines is basically limited by thethickness of the copper foil used on the outer surface of themulti-layer printed circuit board.

[0005] Thus, to produce “densely populated” printed circuit boards, itis necessary to reduce the thickness of the copper, at least on theoutermost surface of the multi-layer printed circuit package. (Thethickness of the copper foil sheet is generally limited by the abilityof a foil manufacturer to handle and transport such sheets. In thisrespect, as the thickness of the foil decreases below 35 μm, the abilityto physically handle such foil becomes more difficult).

[0006] Many printed circuit boards also include conductive layerscontaining patterned components that perform as specific, discretecomponents. One such discrete component is a resistive element. It isconventionally known to form a resistive element using a resistor foil.A resistor foil is basically a copper foil having a thin layer of aresistive material, typically a metal or metal alloy, deposited onto onesurface thereof. The resistor foil is attached to a dielectric substratewith the resistor material being adhered to the dielectric substrate.Portions of the copper foil and resistive material are etched away,using conventionally known etching and masking techniques, to produce atrace line comprised of copper and the resistive material therebelow. Asection of the copper layer is removed leaving only a resistive materialtrace line remaining on the surface of the dielectric to connect the twoseparated ends of the copper portion of the trace line. Because theresistive material typically has a conductivity less than copper, itessentially acts as a resistor between the separated ends of the copperportion of the trace line. As will be appreciated, the foregoingsubtractive procedure requires several masking and etching steps toremove unwanted copper and resistive material to form the actualresistive element. Such steps are both time-consuming and expensive.Further, the resistive materials used in forming the resistor foil aresomewhat limited to those materials that can be etched using knownetching chemicals. In this respect, the resistive material must bematerial that is compatible with chemicals used to etch copper.

[0007] The present invention provides an outer surface component forforming resistive elements in a multi-layer printed circuit board and amethod of forming embedded resistive elements in a multi-layer printedcircuit board that utilizes a process that is not limited by knownresistive materials.

SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to provide a componentfor use in forming multi-layer circuits.

[0009] Another object of the present invention is to provide a componentfor use as the outermost layer of a multi-layer printed circuit, whereinthe component has an exceptionally thin layer of copper that facilitatesfine circuit lines and a “densely populated” circuit surface.

[0010] Another object of the present invention is to provide a componentas described above that has resistive elements thereon for formingembedded resistors within the multi-layered printed circuit.

[0011] Another object of the present invention is to provide a componentas described above that has an exposed copper surface having improvedphotoresist adhesion properties that further facilitates the creation offine circuit lines and a “densely populated” circuit surface by anetching process.

[0012] Another object of the present invention is to provide a componentas described above, wherein one side of the component includes anadhesive layer for attachment to core laminates.

[0013] Another object of the present invention is to provide an outersurface laminate as described above, wherein the outer surface laminateis comprised of a polymeric film having a thin layer of copper adheredto one side of the polymeric film and at least one resistive elementapplied to a second side of the polymeric film.

[0014] These and other objects and advantages will become apparent fromthe following description of preferred embodiments of the invention,taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention may take physical form in certain parts andarrangement of parts, embodiments of which are described in detail inthe specification and illustrated in the accompanying drawings, wherein:

[0016]FIG. 1 is a perspective view of a component for use in forming amulti-layer printed circuit board having embedded resistors,illustrating a preferred embodiment of the present invention;

[0017]FIG. 2 is a perspective view of the component shown in FIG. 1attached to a core showing the component with trace lines formed thereonthat are connected to an embedded resistor;

[0018]FIG. 3 is a cross-sectional view of a multi-layer printed circuitboard formed from components according to the present invention, whereinsuch components form the outermost elements of the circuit board;

[0019]FIG. 4 is a perspective view of a component for use in forming amulti-layer printed circuit having embedded resistors, illustratinganother embodiment of the present invention;

[0020]FIG. 5 is a cross-sectional view taken along lines 5-5 of FIG. 4;and

[0021]FIG. 5A is a schematic representation of the resistive elementshown in FIG. 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0022] Referring now to the drawings wherein the showings are for thepurpose of illustrating preferred embodiments of the invention only, andnot for the purpose of limiting same, FIG. 1 shows a cross-sectionalview of a surface component 10 illustrating a preferred embodiment ofthe present invention. Broadly stated, surface component 10 is comprisedof a polymeric film 12 having a first surface 12 a and a second surface12 b. A thin metallic layer 14 of a flash metal (conventionally referredto as a “tiecoat”) is applied to surface 12 a of polymeric film 12. Atleast one metallic layer 16, preferably formed of copper, is applied toflash layer 14. One or more discrete areas 18 of resistive material areformed on surface 12 b of polymeric film 12. In the embodiment shown, anoptional support substrate 20, that constitutes a discardable element inthe forming of a printed circuit board, is shown attached to metalliclayer 16 along the periphery thereof, to protect the surface of metalliclayer 16 and to provide structural rigidity to component 10.

[0023] Polymeric film 12 is preferably formed of polyimide and has athickness of between 12.5 μm and 125 μm. Specific examples of materialsthat may form polymeric film 12 include Kapton-E or Kapton-HN(manufactured by I. E. DuPont), Upilex-S or Upilex-SGA (manufactured byUbe) and Apical NP (manufactured by Kaneka).

[0024] Flash layer 14 may be formed from metals selected from the groupconsisting of chromium, nickel, titanium, aluminum, vanadium, silicon,iron and alloys thereof. Flash layer 14 is preferably formed of chromiumand preferably has a thickness of between 0 Å (none) and 500 Å, and morepreferably, between about 50 Å to 200 Å.

[0025] As indicated above, metallic layer 16 is preferably formed ofcopper, and has a preferable thickness of between 0.1 μm (1000 Å) and 70μm. The copper forming metallic layer or layers 16 may be applied byvacuum-metallization, electrodeposition, electroless deposition orcombinations thereof on flash layer or layers 14. In accordance with apreferred embodiment of the present invention, metallic layer 16 iselectrodeposited onto flash layer 14.

[0026] Areas 18 are preferably thin layers formed of a material having aresistivity greater than copper. Areas 18 may be formed of a metaldeposited onto surface 12 b by conventionally known deposition processessuch as vacuum-metallization, electrodeposition, electroless depositionor combinations thereof. By way of example, but not limitation, metalsdeposited onto surface 12 b may include chromium, nickel, titanium,aluminum, molybdenum, tantalum, gold, tin, indium, vanadium, silicon,iron and alloys thereof. The thickness of areas 18 is preferably betweenabout 50 Å and about 300 Å. As shall be understood from a furtherreading of the specification, the thickness of areas 18 (as well astheir width and length) will depend upon the desired resultantresistance of the resistive element formed thereby.

[0027] Areas 18 may also be formed of a polymer ink that is sprayed,wiped or painted onto surface 12 b. Resistive polymer inks manufacturedand sold by Metech of Elverson, Pa. may find advantageous application aspart of component 10.

[0028] In the embodiment shown, areas 18 are shown as elongated,rectangular strips of generally uniform width and thickness. As will beappreciated, other shapes may also be used. According to the presentinvention, areas 18 are formed to be discrete areas isolated from eachother.

[0029] Support substrate 20 is provided as a temporary, protectivecovering for metallic layer 16 to protect the outer surface thereof fromcontamination prior to laminating, and further to provide rigidity tocomponent 10 to prevent cracking or flaking of areas 18 resulting frompolymeric film 12 flexing or bending. Accordingly, support substrate 20is preferably dimensioned, i.e., has a thickness, sufficient topreventing cracking or flaking of areas 18. As will be appreciated,different materials forming areas 18 will require different rigiditiesfrom support substrate 20. As indicated above, substrate 20 is removedfrom component 10 and discarded during formation of a printed circuitboard. Substrate 20 is preferably formed of a metal having a polished,substantially contamination-free surface for attachment to metalliclayer 16. Substrate 20 may be formed of aluminum, steel, stainlesssteel, copper or the like. Substrate 20 is attached to the periphery ofmetallic layer 16, typically by a flexible adhesive.

[0030] According to one aspect of the present invention, component 10 ispreferably formed as an individual component for later use in forming amulti-layer printed circuit. Component 10 is preferably used as theoutermost component in a multi-layer printed circuit, wherein metalliclayer 16 forms the outermost layer of the printed circuit.

[0031]FIG. 2 shows a multi-layer printed circuit 30 formed usingcomponent 10 as the outer surface sections thereof. Multi-layer printedcircuit 30 is generally comprised of an inner laminate section 40, thatis shown in phantom in FIG. 2. FIG. 2 shows component 10 after it hasbeen attached, i.e., laminated, to inner laminate section 40 by anadhesive layer 42 and then circuitized by conventionally known processesto form circuit trace lines 52, 54 and 56, 58 on side 12 a of polymericfilm 12.

[0032] More specifically, FIG. 2 illustrates how an embedded resistor 70may be formed using area 18 on side 12 b of component 10. Preferably,the ends of trace lines 56, 58 are disposed in vertical alignment, i.e.,in registry, with the ends of area 18, as illustrated in FIG. 2. Throughholes 62 are drilled into board 30 using conventional techniques, toconnect one end of each trace lines 56, 58 to ends of area 18. Throughholes 62 are filled by conventional, electroplating techniques to form acontinuous circuit comprised of trace lines 56, 58 and area 18. Sincearea 18 is formed of a resistive material, it acts as a resistor elementto current flow from trace line 56 to trace line 58. FIGS. 1 and 2 thusillustrate how an embedded resistor 70 may be formed by an additiveprocess by forming an area 18 of a resistive material onto polymericfilm 12, and then embedding area 18 of a resistive material in a printedcircuit 30 and then connecting opposite ends of area 18 to spaced-aparttrace lines 56, 58 by through holes 62.

[0033] Referring now to FIG. 3, inner laminate section 40 (shown inphantom in FIG. 2) is schematically illustrated in cross-section to showmore clearly the connection between trace line 56, 58 and area 18. InFIG. 3, inner laminate 40 is illustrated as comprised of two previouslyformed printed circuit laminates 80. Circuit laminates 80 are separatedby an intermediate dielectric layer 92. Each printed circuit laminate 80is comprised of an inner core 82 having circuit leads or connectors 84formed on the outer surfaces thereof. As indicated above, cores 82 maybe reinforced or non-reinforced and may include an epoxy, polyester,cyanate ester, bismaleimide triazine, polynorborene, teflon, polyimideor a resinous material, and mixtures thereof, as is conventionallyknown. Printed circuit laminates 80 are secured to dielectric layer 92,as is conventionally known. As shown in FIG. 3, through hole 62 does notextend through adhesive layer 42, although through hole 62 may extendinto adhesive layer 42. FIG. 3 thus illustrates how an embedded resistor70 can be formed using trace lines 56, 58 on the surface of multi-layercircuit 30.

[0034]FIG. 3 also illustrates how component 10 may also be used to forman embedded resistor using internal trace lines. In this respect, FIG. 3shows a lower component designated 10′. Like component 10, component 10′is comprised of a polymeric film 12, a metallic flash layer 14(tiecoat), a metallic layer 16 and at least one area 18′ of a resistivematerial. Flash layer 14 and metallic layer 16 are masked and etched byconventional techniques to form circuit trace lines 96, 98 on surface 12a of polymeric film 12. Area 18′ of a resistive material is oriented anddisposed to be in spaced relationship with circuit leads 84 a, 84 b oncircuit laminate 80. Through holes 62 extending through polymeric film12, area 18′, adhesive layer 42 and into the ends of circuit leads 84 a,84 b, electrically connect circuit leads 84 a, 84 b to the ends of theresistive material of area 18′. Area 18′ thus forms an embedded resistorelement to embedded circuit leads 84 a, 84 b of printed circuit laminate80.

[0035] The resulting multi-layer printed circuit 30 thus has components10, 10′ as the outermost components, with exposed metallic layers 16available for a subsequent etching process to define a specific surfacepath or pattern from metallic layer 16. Importantly, as indicated above,because metallic layer(s) 16 are deposited onto a polymeric film 12, thethickness of metallic layer 16 may be extremely thin as compared toconventional metallic foil. As also indicated above, metallic layer 16may have a thickness as low as 0.1 μm (1000 Å). Such thin layers ofcopper on the outer surfaces of multi-layer printed circuit 30facilitate forming extremely fine and closely spaced circuit lines andpatterns by an etching process. (The exposed, electrodeposited coppersurface of metallic layer 16 is generally rougher than the typicallyflat surface of standard copper foils, thereby providing increasedphotoresist adhesion, which also facilitates forming extremely fine andclosely spaced circuit lines and patterns by an etching process). Asdescribed above, depositing a resistive material onto side 12 b ofpolymeric film 12 facilitates formation of embedded resistors 70 withinmulti-layer printed circuit 30. Unlike prior processes, the presentinvention provides an additive process for forming resistor elements. Anadvantage of the present invention is that the resistive materials thatmay be used in forming resistive elements according to the presentinvention are not limited by their compatibility with etching chemicalsthat are required for forming resistive elements according toconventionally known subtractive processes. Moreover, the absence ofglass fibers (typically found in glass-reinforcing prepregs) makes foreasier laser drilling of microvias and through holes to connect tracelines formed from metallic layer 16 with circuit leads 84 on printedcircuit laminates 80 or resistive areas 18. Still further, polymericmaterials, such as polyimide, have better dielectric properties ascompared to conventional glass-reinforced prepregs, thereby providingimproved electrical performance, such as for example, reducedattenuation of high speed signals. Furthermore, the high heat stabilityof materials such as polyimides can provide better resistance to thermalexpansions that arise during the chip attachment process. Thus,components 10, 10′ as used as an outer surface layer in a multi-layerprinted circuit assembly, facilitate a formation of embedded resistors70 by an additive process, as well as the production of more denselypacked multi-layer printed circuit boards.

[0036] A contemplated method of forming an embedded resistor within aprinted circuit would be as follows:

[0037] 1) Forming a component 10 as described above, comprised of apolymeric film 12 having on one side thereof a flash layer 14 of atiecoat metal and a metallic layer 16 deposited on flash layer 14, andon the other side thereof, discrete, isolated areas 18 of resistivematerial. A support substrate 20 may optionally be provided to protectthe exposed surface of metallic layer 16. Substrate 20 may also beprovided to prevent flexing or bending of component 10 so as to preventcracking or separating of certain types of resistive materials formingareas 18.

[0038] 2) Laminating component 10 to an inner laminate by means of anadhesive, wherein areas 18 are embedded within the resulting componentand separated from the inner laminate 40 by an adhesive layer.Lamination of component 10 to an inner laminate 40 comprises compressingcomponent 10 together with elements forming inner laminate 40 underconditions of heat and pressure to create a multi-layer printed circuit.

[0039] 3) Removing support substrate 20 so as to expose metallic layer16 and circuitizing metallic layer 16 by conventionally known maskingand etching processes to form circuit trace lines from metallic layer 16and flash layer 14.

[0040] 4) Drilling through holes through the ends of spaced-apart tracelines, the through holes extending through polymeric film 12 into remoteportions of areas 18.

[0041] 5) Plating or filling the through holes with conductive materialto create an electrical connection between the ends of trace linesformed on the outer surface of component 10 and the embedded areas 18 ofa resistive material, so as to form a resistive element.

[0042] The foregoing description is a specific embodiment of the presentinvention. It should be appreciated that this embodiment is describedfor the purpose of illustration only, and that numerous alterations andmodifications may be practiced by those skilled in the art withoutdeparting from the spirit and scope of the invention. For example, FIGS.4-5A show a component 110 illustrating another embodiment of the presentinvention. Component 110 is similar to component 10 in that it includesa polymeric film 12 having a first surface 12 a and a second surface 12b. A flash layer 14 of a tiecoat metal is applied to surface 12 a, andmetallic layer 16 is applied to flash layer 14. As heretofore described,component 110 is similar to component 10 and therefore like elementshave been designated with like reference numbers. In the embodimentshown, discrete areas 118 of overlapping resistive materials 118 a and118 b are formed on side 12 b of polymeric film 12. Areas 118 may beformed of overlapping metal layers of the type heretofore described, ormay be comprised of overlapping layers of a polymer ink of the typeheretofore described. Preferably, each layer 118 a is different fromlayer 118 b and has different resistive characteristics.

[0043] In a manner similar to that described above, component 110 islaminated as part of a multi-layer printed circuit to an inner corelaminate 140. Through holes 162 connect the ends of resistive areas 118to trace lines 132, 134 formed on the outer surface 16 of component 110.As described above, an embedded resistor is formed as a result of area118 connecting trace lines 132, 134. Because of the overlapping regionof area 118, the resultant embedded resistor has a resistive equivalentto that schematically illustrated in FIG. 5A. FIGS. 4-5A thus illustratehow various types of resistive components can be formed by overlayingmaterials having different resistive characteristics.

[0044] It is intended that all such modifications and alterations beincluded insofar as they come within the scope of the invention asclaimed or the equivalents thereof.

Having described the invention, the following is claimed:
 1. A method offorming resistive elements in a multi-layer printed circuit, comprisingthe steps of: a) forming an inner core from one or more printed circuitlaminates, each of said printed circuit laminates having a coresubstrate and a first surface with at least one strip conductor disposedthereon, b) forming at least one surface laminate, said surface laminatecomprised of: a film substrate formed of a first polymeric material; atleast one layer of a flash metal applied to a first side of said filmsubstrate; at least one layer of copper on said layer of flash metal;and a discrete area of a resistive material formed on a second side ofsaid film substrate; c) applying an adhesive material between saidsurface laminate and said inner core, d) compressing said inner core andsaid surface laminate together under conditions of heat and pressure tocreate a first multi-layer printed circuit, wherein said discrete areaof resistive material is embedded within said first multi-layer printedcircuit between said film substrate and said adhesive layer; e)circuitizing said layer of copper on said surface laminate to form atleast one strip conductor thereon; f) connecting an end of a first stripconductor with a first end of said resistive area by a through holeconnection; and g) connecting an end of a second strip conductor with asecond end of said resistive area by a through hole connection.
 2. Amethod as defined in claim 1, wherein said at least one flash layer iscomprised of a metal selected from the group consisting of nickel,chromium, titanium, aluminum, iron, vanadium, silicon and alloysthereof.
 3. A method as defined in claim 2, wherein said at least oneflash layer has a thickness of about 50 Å to about 500 Å.
 4. A method asdefined in claim 3, wherein said at least one layer of copper has athickness of about 1000 Å to about 35 μm.
 5. A method as defined inclaim 4, wherein said first polymeric material is a polyimide.
 6. Amethod as defined in claim 5, wherein said adhesive layer is formed froma material selected from the group consisting of acrylics, epoxies,nitrile rubbers, phenolics, polyamides, polyarylene ethers,polybenzimidazoles, polyesters, polyimides, polyphenylquinoxalines,polyvinyl acetals, polyurethanes, silicones, vinyl-phenolics,urea-formaldehyde and combinations thereof.
 7. A method as defined inclaim 1, wherein said resistive material is a metal or metal alloyhaving a resistivity greater than copper.
 8. A method as defined inclaim 7, wherein said metal is selected from the group consisting ofchromium, nickel, titanium, aluminum, vanadium, silicon, iron and alloysthereof.
 9. A method as defined in claim 1, wherein said resistivematerial is a polymer resistor ink.
 10. A multi-layer printed circuit,comprising: a) an inner core formed from one or more printed circuitlaminates, said printed circuit laminates comprised of a core substratehaving a first surface with a strip conductor disposed thereon, b) atleast one surface component attached to said inner core, said surfacecomponent, comprised of: a film substrate formed of a first polymericmaterial; at least one layer of copper on one side of said polymericmaterial; and a discrete area of a resistive material disposed on asecond side of said film substrate, said surface laminate attached tosaid inner core with said discrete area of resistive material embeddedwithin said multi-layer printed circuit between said core and said filmsubstrate, c) a first through hole connecting one end of said discretearea to a first circuit trace line of said multi-layer printed circuit;and d) a second through hole connecting another end of said discretearea to a second trace line of said multi layer printed circuit.
 11. Amulti-layer printed circuit as defined in claim 10, wherein saidresistive material is a metal or metal alloy having a resistivitygreater than copper.
 12. A multi-layer printed circuit as defined inclaim 11, wherein said metal is selected from the group consisting ofchromium, nickel, titanium, aluminum, vanadium, silicon, iron and alloysthereof.
 13. A multi-layer printed circuit as defined in claim 10,wherein said resistive material is a polymer resistor ink.
 14. Amulti-layer printed circuit as defined in claim 10, wherein at least onelayer of a flash metal is disposed between said polymeric material andsaid at least one layer of copper.
 15. A component for use in forming amulti-layer printed circuit comprised of: a film substrate formed of afirst polymeric material; at least one layer of a flash metal applied toa first side of said film substrate; at least one layer of copper onsaid layer of flash metal; and a discrete area of a resistive materialdisposed on a second side of said film substrate.
 16. A component asdefined in claim 15, further comprising a metal support substrate thatconstitutes a discardable element in the formation of a printed circuitboard, one surface of said metal support substrate being essentiallyuncontaminated and engageable with said layer of copper, said supportsubstrate attached to said layer of copper at its borders to define asubstantially uncontaminated central zone of copper inwardly of theedges of the copper layer.
 17. A component as defined in claim 15,wherein said resistive material is a metal or metal alloy having aresistivity greater than copper.
 18. A component as defined in claim 17,wherein said metal is selected from the group consisting of chromium,nickel, titanium, aluminum, vanadium, silicon, iron and alloys thereof.19. A component as defined in claim 15, wherein said resistive materialis a polymer resistor ink.
 20. A component as defined in claim 15,wherein said discrete area of resistive material is dimensioned to beattached to an inner core of a multi-layer printed circuit board withsaid discrete area of resistive material embedded within saidmulti-layer printed circuit between said core and said film substrate.